the fish were caught one at a time
the guy used a piece of rope as a stringer
he tied it to the first fish, when the second fish was caught he slide the rope through the gills and out through the mouth
over time, while the stringer held the fish in the water, the first fishes head got stuck in the gill plate of the second fish on the stringer giving it the look you see in the picture
it is an optical illusion..... 2 fish no mutation
but it does make one hell of a internet story